1. Field of the Invention
The present invention relates to a remote-controlled circuit breaker and more particularly to a remote-controlled circuit breaker having improved durability and reliability.
2. Description of the Related Art
FIG. 11 is a schematic diagram showing a conventional driving circuit for a three-phase induction motor M. An A.C. voltage is applied to the motor M through electric power lines (not shown) via a conventional circuit breaker 1 and a magnetic contactor 2 connected in series with each other. The circuit breaker 1 is provided primarily to protect the motor M and connection wires 4 from heat damage caused by a short-circuit or an overload. Since the rated lifetime of the contacts of a conventional circuit breaker is generally under 10,000 switching cycles, the circuit breaker 1 is not suitable for making/breaking its contacts with high frequency. Further, it is difficult to effect remote-control operation of a conventional circuit breaker such as the circuit breaker 1.
On the other hand, the magnetic contactor 2 is suitable for making/breaking its contacts with fairly high frequency. If only the magnetic contactor 2 were used to drive the motor M, however, without the series-connected circuit breaker 1, welding of contacts in the magnetic contactor 2 could occur when a large current flows through the contacts as a result, for example, of a short circuit, rendering the magnetic contactor useless. For the above-mentioned reasons, the circuit breaker 1 and the magnetic contactor 2 are connected in series with each other, thereby realizing both a breaking function in response to an excessive current and a high-frequency making/breaking function susceptible to remote control.
As shown in FIG. 12, both the circuit breaker 1 and the magnetic contactor 2 are conventionally fixed to a common casing 3 to constitute a protection and control unit.
However, since the circuit breaker 1 and the magnetic contactor 2 are separate devices, many interconnecting wires 4 are necessary in the casing 3. In order to provide sufficient space to accommodate both devices (the circuit breaker 1 and the magnetic contactor 2), the interconnecting wires 4, and various wire connecting devices, the casing 3 must unavoidably be large.
A second shortcoming in an arrangement such as that shown in FIG. 11 is that there is no coordination or cooperation between the operation of the circuit breaker 1 and the magnetic contactor 2. Since these devices are in series, however, it is possible that the magnetic contactor 2 might operate needlessly to try turn the motor on and off when the circuit breaker 1 has already cut off power. The resultant useless motion of the electromagnet of magnetic contactor 2 causes unnecessary mechanical shock resulting in needless wear which shortens the service life of the overall protection and control unit.